Ground-based antenna systems are commonly used for providing communications with moving aircraft. Transmit power and antenna gain at a ground-based antenna system are sufficient to overcome normal spreading attenuation losses as well as ambient background noise levels.
In addition to overcoming normal attenuation losses and ambient background noise levels, ground reflections also present a problem. Ground reflections may cause deep radiation pattern ripples and fades, as illustrated by the elevation plane radiation plot 20 in FIG. 1. Reflections from the ground of a transmitted communications signal cause constructive and destructive interference, which results in an elevation plane radiation pattern with peaks 22 and intervening deep ripples above the horizon. The valleys or nulls 24 within the ripples cause the antenna gain to be significantly reduced. Ideally, an elevation plane radiation pattern without ripples would be obtained if the ground reflections were not present. A particularly deep null or “ground tuck” appears in the radiation patterns at the horizon for both vertical and horizontal polarization.
One approach to filling the nulls within a reflected communications signal is to increase the effective isotropic radiated power (EIRP) of the ground-based antenna system. In most licensed frequency bands, the transmit power and/or antenna gain may be increased as needed. However, this is not permissible in some bands, such as the Industrial Scientific And Medical (ISM) band.
Another approach to account for ground reflections is to use an iterative clutter calibration method as disclosed in U.S. Published Patent Application No. 2011/0241931, which measures an average of a sidelobe power in a range-Doppler image for a plurality of ranges. A determined value of an objective function is responsive to an average of the sidelobe clutter power. A plurality of beamformer weights is modified and the step of determining the value of the objective function is repeated until a maximum value of the objective function is determined. Each beamformer weight determines a gain and phase of a respective antenna element in an antenna system.
However, when operating within the unlicensed industrial, scientific and medical (ISM) radio frequency (RF) band, the FCC places restrictions on transmit power and antenna gains of devices operating within this band. One of the ISM RF bands is within a frequency range of 2.4 GHz to 2.4835 GHz, and is reserved for industrial, scientific and medical purposes other than telecommunications. Example ISM applications include RF process heating, microwave ovens and medical diathermy machines.
In recent years, the fastest-growing use of this band has been for short-range, low power communications systems. For instance, low power communications devices operating within this frequency band include Wi-Fi devices, cordless phones, Bluetooth devices, near-field communication (NFC) devices and wireless computer networks. As a result of the proliferation of these short-range, low power communications systems, ground clutter within this ISM RF band has significantly increased.
Consequently, there is a need to reject this ground clutter as well as ground reflections when communicating within the ISM RF band using a ground-based antenna system and a moving aircraft. The difficulty is to overcome the ground clutter and ground reflections without increasing the EIRP of the ground-based antenna system beyond the limits imposed by the FCC part 15 rules.